1. FIELD OF THE INVENTION
The present invention relates to lasers which provide an output beam with high power and high optical quality and more particularly to unstable ring resonators having spatial filters.
2. DESCRIPTION OF THE PRIOR ART
Laser systems which provide an output beam at a very high power level such as those made possible with combustion driven chemical lasers are subject to various restraints which limit the actual output power from such a system and which limit the optical quality of the laser beam. An unstable ring resonator capable of simultaneous optimization of the transverse mode pattern of a beam within the resonator for high optical quality together with mode volume control for high power output is disclosed by Buczek et al in U.S. Pat. No. 3,824,487 entitled "Unstable Ring Laser Resonator" filed May 8, 1972 and held with the present application by common assignee. The resonator described therein involves separating the parameters which control the transverse mode discrimination of the resonator from those parameters which control the fractional output coupling and mode volume of the laser system. The unstable resonator includes a first region in which a beam of electromagnetic radiation passing therethrough is noncollimated and a second region in which a beam passing therethrough is collimated. The first region is formed with at least two curved reflection surfaces which permit the size of the beam to be changed within the resonator and is capable of providing a collimated beam to the second region. The energy density of the collimated beam is enhanced by passing the beam through a gain medium in the second region and a high power laser output beam is produced by diffractively coupling a portion of the enhanced beam out of the resonator as a collimated output beam having an annular shape. The remaining portion of the enhanced beam passes into the first region where it is magnified and recycled.
Another approach to the problem of obtaining high quality optical output beams having high power is disclosed by Freiburg et al in the U.S. Pat. No. 3,969,688 entitled "Traveling Wave Unstable Resonators for Radial Flow Lasers" filed Apr. 14, 1975 and held with the present application by a common assignee. The unstable ring resonator is formed having a centerline axis with a gain medium in the configuration of a thin wall cylinder disposed symmetrically about the centerline axis. The volume of the gain medium is determined by the maximum height of the cylindrical sheath which can be accommodated in the resonator without incurring superfluorescence. Since the volume of the gain medium can be increased by simply increasing the inside diameter of the cylindrical sheath, the volume is essentially unlimited and capable of providing output beams having high power. High optical quality of the beam results from the effective discrimination against high order transverse modes provided by a region of common resonance dominated by diffraction cross coupling.
A method of improving the optical quality of a laser beam produced by an unstable ring resonator is to provide spatial filtering of the beam within the resonator to filter out undesirable modes. A negative branch confocal optical system incorporated within a transverse mode discrimination region of a ring laser includes an optical system having a spherical mirror to focus a circulating beam to a diffraction limited spot and a second spherical mirror to collect the radiation expanding from the focused spot to provide a collimated beam to the active gain region of the resonator. The collimated beam has a diameter corresponding to the diameter of the second spherical mirror. A spatial filter having a circular aperture is located at the common focal plane of the optical system to provide filtering of unwanted high order modes. This concept is feasible for low power lasers where the power density in the focal point is sufficiently low to allow spatial filtering. However, in high power lasers, the power density at the focal point of the optical system is sufficiently high to destroy spatial filters made with known materials and utilizing known cooling techniques.